Forests, grasslands and shrublands in the West sequester nearly 100 million tons of carbon each year, an amount equivalent to counterbalancing the emissions of about 83 million passenger cars a year in the United States, according to a new USGS report.

Carbon that is absorbed or â€śsequesteredâ€ť through natural processes reduces the amount of carbon dioxide in the atmosphere.

While the study showed that western ecosystems are a strong carbon sink now, the region could experience a decline in storage potential between now and 2050, depending on future changes in land-use, climate and wildfires. Future carbon stocks are inextricably linked to these drivers because as ecosystems, forests or agricultural lands are converted for other uses, their ability to capture and store carbon is affected.

From the Rocky Mountains to the Pacific Coastal Waters

The area USGS studied extends from the Rocky Mountains to the Pacific coastal waters, and totals just over 1 million square miles. The major ecosystems evaluated were terrestrial — forests, wetlands, agricultural lands, and shrublands and grasslands — and aquatic — rivers, lakes, estuaries and coastal waters. It includes well-known ecosystems, such as the Rocky Mountains and the Sierra Nevada Mountains, the Mojave and Sonoran deserts, the Pacific Northwest forests and the vast grasslands and shrublands of the Great Basin.

Western Forests Stored the Most Carbon

While the western ecosystems varied widely in their potential for storing carbon now and in the future, forests are by far the largest carbon-storing pools, accounting for about 70 percent of the carbon stored recently in the West.

Forests cover 28 percent of the land areas of West, contain the most carbon per unit area, and have the second-highest rate of sequestration of ecosystem types.

Wetlands cover less than 1 percent of the West andhad the highest rate of sequestration of all ecosystem types, but because they cover only such a small percentage of land, the amount of carbon they sequester is far less significant than other ecosystem types.

Grasslands and shrublands cover nearly 60 percent of the West and contain 23 percent of the regionâ€™s carbon stored recently.

Agricultural lands coverabout 6 percent of the land areas of the West and contain 4.5 percent of the carbon stored recently.

Significant Greenhouse Gas Emission Sources in Western Ecosystems

Wildland fires in western ecosystems generated significant amounts of greenhouse gas emissions, with such emissions equivalent to 13 percent of the estimated rate of the recent annual carbon sequestration by western terrestrial ecosystems. This amount could increase up to 31 percent in the future.

Water bodies in the West emitted even more CO2 than fires. Emissions from water bodies are equivalent to more than 30 percent of the recent annual carbon sequestration rate of terrestrial ecosystems in the West.Â Basically, the more interaction with the atmosphere, the more CO2 is released.Â So, in fast-moving waters, where the water is churned up, there is a greater loss of CO2 to the atmosphere.

Land-Use, Land-Cover, and Carbon Stocks

Future changes in the ability of western ecosystems to sequester carbon will depend on future changes in land-use, climate, and wildfires. Â Future carbon stocks are tied to these drivers because as ecosystems, forests or agricultural lands are converted for other uses, their ability to capture and store carbon is affected. Â Land use by people causes a significant loss of carbon from ecosystems. Specific examples are forest harvesting (nearly 13 million tons of carbon per year) and agricultural harvesting (more than 20 million tons of carbon per year).